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  #586  
Old 01-13-2009, 09:32 PM
Guest625101138 Guest625101138 is offline
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I am an electrical engineer and I am never too surprised by the lack of real understanding people have of the physical world.

I once got into a lengthy debate with 4 mechanical engineers that the torque through a slip coupling was conserved. (Maybe a very small allowance for windage on the coupling body) They could not figure out how I determined the power loss in the coupling simply by knowing the input power and speed from the the driving motor and the output speed of the driven fan. They wanted me to show them the power calculation for the fan because they figured power loss was input power less output power. It is of course but there is more than one way of arriving at the answer.

So it takes a lot to surprise me when it comes to poor appreciation of the physical world by educated professionals.

The problem with Mark's material is that it has not been peer reviewed (at least yet) and published in a respected journal or conference volume. Some of the self professed experts that visit here require this as they do not have the ability to understand mathematical information provided. I am sure some turn off as soon as they see an algebraic equation. I do not mind this but then they do not bother to seek understanding; just continue to offer an illfounded opinion.

Rick W
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  #587  
Old 01-13-2009, 09:45 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by spork View Post
...

You can't even begin to imagine. Much to my own embarrassment, the worst case that comes to mind is an instructor at my own alma mater (GA Tech).
Actually I thought my four slides that go through the propeller and generator was compelling if you could get people to work through it. Please use it if you think it would help. It needs to be done in a way that gets people engaged. Probably one slide at a time. You know - will this work? The second last step is to get rid of the battery and finally replace all the electrics with mechanical transmission.

(Although from Guillermo's perspective propellers can never exceed the velocity of the fluid they are operating in. Someone needs to tell all those boats and planes that move through fluids faster than the fluid using a propeller that they are not obeying Guillermo's law of fluid dynamics.)

Rick W
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  #588  
Old 01-13-2009, 09:45 PM
spork spork is offline
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Quote:
Originally Posted by Rick Willoughby View Post
...So it takes a lot to surprise me when it comes to poor appreciation of the physical world by educated professionals.
I'll grant you that not understanding that a slip coupling has no way to absorb torque is particularly shameful for mechanical engineer, but I've debated endlessly as to whether the wind moving over the ground is the same as the ground moving under the air. I would hope people could accept this as simply one of the most basic principles of physics - with no need for further CPU usage. That we have PhD's and instructors that insist DDWFTTW is not possible is bad, but not accepting the equivalence of inertial frames is almost unthinkable.

Quote:
The problem with Mark's material is that it has not been peered reviewed...
Fair enough. As you point out, this is valid for people that trust the process rather than trusting their own ability to understand the analyses. I'm sure I fall into that category for a great many things.
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  #589  
Old 01-13-2009, 09:48 PM
spork spork is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Actually I thought my four slides that go through the propeller and generator was compelling if you could get people to work through it.
Your approach is very similar to one that JB has used. I've used a number of approaches that I consider essentially bulletproof. The problem seems to be that many sceptics start with the conclusion that DDWFTTW is impossible, and measure all evidence relative to that assumption. Many of them simply will not engage.
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  #590  
Old 01-14-2009, 12:32 AM
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Guillermo Guillermo is offline
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Quote:
Originally Posted by Rick Willoughby View Post
Guillermo
I take back what I said earlier - you should eat your hat. It might do you some good.

You are talking about a DDWFTTW cart and turbine again. WRONG. You need to eat your hat to get the message. IT IS ALWAYS A PROPELLER AS LONG AS THE DDWFTTW CART IS MOVING.

If it is a turbine you have an upwind cart going against the wind with the turbine powering the wheels.
I think you should read my post twice.


Quote:
Originally Posted by Rick Willoughby View Post
I guess we could all accept your poorly put, confusing explanation that it cannot happen and the 20 or so who actually appreciate the science involved just accept your superior understanding of all things physical and agree with you to save the aggravation.

Rick W.
I think your style is quite disgusting and offensive. Do you think that defends better your ideas?

Cheers.
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Guillermo Gefaell
Gestenaval S.L., Naval Architecture & Marine Engineering
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  #591  
Old 01-14-2009, 01:59 AM
tspeer tspeer is offline
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Originally Posted by robherc View Post
...If you figure that the amount of propulsion of any propeller results from its ability to "push" air towards the opposite direction as the propeller is attempting to travel, it makes this easier....
May I suggest it's helpful to think of a helicopter analogy. There are three different situations the rotor can be in. When the helicopter is climbing or in a level hover, the air moves downward through the rotor disk and is accelerated by the rotor, producing the lift that supports the helicopter's weight.

When the helicopter is descending rapidly, the air is passing through the rotor disk from bottom to top. However, the rotor decelerates the air, so that again the force is in the upward direction.

In between is the vortex ring state, obtained when the helicopter is descending moderately slowly. The air moves up through the rotor disk in the center, and down through the rotor disk toward the edge. Accompanied by turbulence and ineffective production of lift. But lifting nonetheless.

The Bauer machine just starting is like the autorotating helicopter. The air is moving from the back of the rotor to the front. The drag moves the machine forward, and the wheels apply torque to the rotor. However, there may be a positive aerodynamic torque from the autorotating rotor as well. If the torque from the wheels is greater than the aerodynamic torque, then the wheels can be said to be powering the rotor. If the aerodynamic torque is greater than the torque from the wheels, then the rotor may actually be powering the wheels. But the difference is really not very significant because the rotor is still producing forward thrust in either case. The aerodynamic torque comes from the fact that the lift vector is tilted forward relative to the blade chord, just like that of a glider whose descending trajectory tilts its lift vector forward and opposes the horizontal component of its drag.

As the Bauer machine picks up speed, it is like a helicopter pulling out of a vertical autorotating descent. Power is applied to the rotor and its velocity slows relative to the air, and then reverses. The vortex ring state forms and then shrinks toward the hub as more of the air flows from front to back through the rotor, until the air is flowing from front to back all along the rotor disk. Initially, this will be due to the induced velocity from the rotor's thrust as the machine has not quite yet reached the freestream wind velocity. Some parts of the inner disk may be producing an autorotating torque, but the outer portions are not, requiring the application of external power to keep the rotor turning. The wheels are powering the rotor by this point and the rotor can no longer be viewed as a windmill, if it ever was.

But as the machine continues to accelerate, it reaches and exceeds the velocity of the air, like a helicopter that finally starts to gain altitude. Now the velocity through the rotor disk is the sum of the machine's relative wind and the induced velocity. There is no doubt at this point that it is the wheels that are powering the rotor and not the other way around, because the lift vector, which is perpendicular to the vector sum of the rotational speed and the inflow, is tilted back and has a component in the plane of the disk that would retard the blade if it were not for the torque being applied at the hub.

Throughout, thrust from the rotor keeps the machine moving forward. Only in the beginning can the torque to the wheels be adding to the rotor thrust. As the craft accelerates, the wheel torque lessens and then reverses, acting as a drag on the vehicle. But as long as the thrust is greater than the sum of the aerodynamic drag and the drag from the wheels, the craft will accelerate. Eventually, the velocity through the rotor disk will become so high that the lift vector will be tilted back too far and the aerodynamic drag of the rotor will acquire a backward (upwind) component, and the thrust will drop off. When the thrust matches the drag from the wheels, the craft will have achieved steady-state operation.

Power is a tricky thing to work out for any sailing craft, because the power available changes. Power is thrust times velocity, so at rest the thrust horsepower is zero, even though the thrust is not. As the yacht accelerates, the thrust horsepower increases because the thrust increases as the rotor becomes more efficient (especially if the pitch is fixed), and because the velocity is increasing. As the thrust starts to drop, power still increases, but more slowly. (Just like the peak power occurs at a higher rpm than the peak torque of your car's engine.) But the power extracted by the wheels is also increasing because of both the increasing torque demands of the rotor and the higher speed of the machine. Eventually, the two curves come together, and the power balance is achieved at steady state. Either the energy flows (power) or force & moment sums can be used to calculate what's happening, but as a practical matter, it's easier to use the forces & moments.

So while a landyacht is like a fixed wing glider soaring in a thermal, the Bauer machine is like a helicopter, powered by the transmission from the wheels.
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  #592  
Old 01-14-2009, 02:31 AM
spork spork is offline
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Tom, what you say about the Bauer cart is true if we know that he was able to actually reverse his prop pitch. We can see from the pictures that he can adjust prop pitch, but I'm not sure if anyone knows whether it can be reversed. If not, then the wheels always apply the torque that turns the propeller. This is the case with the carts we've built. The prop pitch is fixed, but they still self start without any trouble.
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  #593  
Old 01-14-2009, 03:27 AM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by spork View Post
Tom, what you say about the Bauer cart is true if we know that he was able to actually reverse his prop pitch. We can see from the pictures that he can adjust prop pitch, but I'm not sure if anyone knows whether it can be reversed. If not, then the wheels always apply the torque that turns the propeller. This is the case with the carts we've built. The prop pitch is fixed, but they still self start without any trouble.
Spork
You are correct about the pitch. It has to be quite aggressive such that for each revolution it wants to travel further than the cart can travel. Thus the blades have a large pitch angle compared with those of a helicopter. It is by virtue of the mechanical advantage being in favour of the wheels that the cart goes downwind.

If the rotor pitch is is less than the distance the vehicle travels in one revolution of the rotor then the rotor becomes a turbine and the vehicle goes upwind with the turbine torque overcoming the opposing torque from the wheels. The direction of travel relative to the direction of wind over the ground is based on what has mechanical advantage.

Actually I have noted that in the road test video of your cart there is a very small reverse snap of the prop before it starts to power forward. This is not evident in the fan from behind test. I was wondering if there was backlash in the gears or if it actually caused the wheels to skid.

Rick W
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  #594  
Old 01-14-2009, 03:41 AM
spork spork is offline
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Quote:
Originally Posted by Rick Willoughby View Post
If the rotor pitch is is less than the distance the vehicle travels in one revolution of the rotor then the rotor becomes a turbine and the vehicle goes upwind with the turbine torque overcoming the opposing torque from the wheels. The direction of travel relative to the direction of wind over the ground is based on what has mechanical advantage.
This is right on the money, except I think you misspoke regarding the ratio of pitch to cart travel. The prop pitch divided by the distance the cart rolls forward in a single rotation of the prop is what I term the "advance ratio" for the cart. For a DDWFTTW cart the advance ratio has to be less than 1.0 (the prop pitch must be LESS than the distance the cart rolls foward). With an advance ratio greater than on 1.0 we get an upwind cart with the prop acting as a turbine.

Quote:
Actually I have noted that in the road test video of your cart there is a very small reverse snap of the prop before it starts to power forward. This is not evident in the fan from behind test. I was wondering if there was backlash in the gears or if it actually caused the wheels to skid.
As you noted above, the prop contends with two opposing torques. The wind would tend to move it CCW (from behind) while the torque on the shaft (from the wheels) tends to turn it CW. In the test you're describing, the cart was subjected to a sudden wind gust, and we did not have the rubber bands on the wheels to provide the extra traction for that run. As a result, the wheels were actually caused to spin backwards for the first few seconds. As you can see, this doesn't happen with a more moderate breeze to start the cart. In fact we've allowed it to self-start outdoors several times, and have only seen that once.
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  #595  
Old 01-14-2009, 04:23 AM
Guest625101138 Guest625101138 is offline
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spork
You are correct. I have it reversed.

For the cart I detailed in the DDWFTTW thread, post #31, the prop would be advancing at 2.7m/s if in an unloaded state when the vehicle is doing 5m/s. This is the zero apparent wind condition but the actual airflow through the disc is 2.3m/s. This is for the 3.5m asymmetric foil.

Rick W
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  #596  
Old 01-14-2009, 12:04 PM
robherc robherc is offline
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Guillermo:

Will you please make comment on my posts. I understand that you don't like Rick's posts, but I have done nothing to offend you, and have provided the information you requested, but receive no response?
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  #597  
Old 01-14-2009, 12:26 PM
spork spork is offline
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Quote:
Originally Posted by robherc View Post
Will you please make comment on my posts. I understand that you don't like Rick's posts, but I have done nothing to offend you, and have provided the information you requested, but receive no response?

Don't take it personally Rob. That's his M.O. I've also tried to engage him in a discussion to reach a genuine understanding, as has JB. Good luck.
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  #598  
Old 01-14-2009, 03:53 PM
robherc robherc is offline
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I know, but I had to give it "one last shot" ... just hate giving up on people who've shown promise elsewhere.
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  #599  
Old 01-14-2009, 04:17 PM
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chabrenas chabrenas is offline
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Rick, your description of faster-than-the-wind sailing directly downwind still sounds like perpetual motion to me. I'll have to go back and revise my understanding of the Second Law of Thermodynamics.

There is no flow through the water turbine until the wind starts to push the craft downwind - partly through drag on the whole structure and partly through work done on the wind turbine - how does the system switch over from being air-driven to water-driven? The wheel-driven land system is the same - what drives the wheels? I'll sleep on it and come back tomorrow with a rigorous analysis.
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  #600  
Old 01-14-2009, 04:47 PM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by chabrenas View Post
Rick, your description of faster-than-the-wind sailing directly downwind still sounds like perpetual motion to me. I'll have to go back and revise my understanding of the Second Law of Thermodynamics.

There is no flow through the water turbine until the wind starts to push the craft downwind - partly through drag on the whole structure and partly through work done on the wind turbine - how does the system switch over from being air-driven to water-driven? The wheel-driven land system is the same - what drives the wheels? I'll sleep on it and come back tomorrow with a rigorous analysis.
If you desire is to understand rather than just stick with the notion it defies basic physics then I will step you through it. If you have the correct understanding of the second law of thermodynamics you will not need to revise it. The understanding will take 5 exchanges and there is no trick.

First one is in the attached slide. The questions you need to answer are:
1. Will this vehicle actually work?
2. Are the numbers realistic for a vehicle weighing 20kg?

Rick W
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The Wind Powered Sail-less Boat-slide1.png  
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